1. Field of the Invention
The present invention relates to a technical field of a hydraulic brake boosting system for boosting depressing force applied to a brake pedal of, for example, an automobile, with power generated by hydraulic pressure of operating fluid so as to enlarge braking force, and more particularly to an abnormal condition detecting apparatus for a hydraulic brake boosting system for detecting an abnormal condition of a pump-operation-control apparatus for operating and controlling a pump for generating hydraulic pressure of the operating fluid and restraining the driving speed if an abnormal condition is detected.
2. Description of the Related Art
Brake boosting systems for vehicles, for example, automobiles, include a hydraulic brake boosting system which incorporates a hydraulic brake boosting apparatus for boosting the force for depressing a brake pedal so as to enlarge the braking force. The hydraulic brake boosting system enables great braking force which cannot be obtained from only the depressing force which is applied to the brake pedal. Moreover, the force for depressing the pedal can be reduced.
An hydraulic brake boosting system of the foregoing incorporates a pump which is operated by a motor so as to generate hydraulic pressure of the operating fluid and an accumulator for accumulating the generated hydraulic pressure. When the brake pedal has been depressed, the hydraulic pressure accumulated in the accumulator is introduced into the hydraulic brake boosting apparatus. Thus, the hydraulic brake boosting apparatus produces an output obtained by boosting the depressing force applied to the pedal in accordance with the depressing force. The thus-produced output causes a master cylinder to be operated.
The above-mentioned brake boosting system has a necessity that hydraulic pressure of a predetermined level can assuredly be introduced into the hydraulic brake boosting apparatus when the brake is operated. Therefore, the operation of the pump must be controlled in such a manner that predetermined hydraulic pressure is always accumulated in the accumulator.
An alarm-issuing apparatus for a conventional pump operation control apparatus for controlling the operation of the pump as described above has been disclosed in Japanese Patent Laid-Open No. 7-33002. According to the disclosure, a pump-operating pressure sensor (or a pressure sensor which is hereinafter and representatively called a "pressure sensor") is employed to hydraulic pressure accumulated in the accumulator. When the hydraulic pressure accumulated in the accumulator has been raised to a level with which the pump must be operated, the pump is operated to raise the hydraulic pressure accumulated in the accumulator. Moreover, an alarm-issuing pressure sensor is employed to detect and issue an alarm when the hydraulic pressure accumulated in the accumulator has abnormally been reduced.
The alarm-issuing apparatus disclosed as described above is arranged to issue an alarm when the hydraulic pressure accumulated in the accumulator has been made to be lower than a predetermined level. Moreover, an alarm delay circuit is provided to prevent a malfunction so as to prevent incorrect issue of an alarm when a vehicle allowed to stand for long time is started in a state in which the hydraulic pressure accumulated in the accumulator is at a low level.
However, the above-mentioned alarm-issuing unit disclosed as described above is arranged such that an alarm is issued only after the hydraulic pressure accumulated in the accumulator has been made to be lower than a predetermined level. Therefore, the alarm-issuing pressure sensor is not operated if the hydraulic pressure accumulated in the accumulator is not lower than the predetermined level. Thus, an abnormal condition of the pump, the motor or the sensor cannot be detected. Moreover, when an alarm has been issued from the alarm-issuing pressure sensor and thus an abnormal condition of the pump, the motor or the sensor has been detected, the hydraulic pressure accumulated in the accumulator has been reduced. Therefore, there is apprehension that the braking force becomes insufficient when the vehicle is driven at high speed or on a slope.
Moreover, the characteristics of the pressure sensor are sometimes changed because of deterioration or the like. If the characteristics of the pressure sensor are changed, the hydraulic pressure level accumulated in the accumulator for interrupting the operation of the pump is undesirably changed. Thus, the hydraulic pressure level accumulated in the accumulator becomes insufficient to obtain satisfactorily large braking force or the hydraulic pressure level accumulated in the accumulator is raised excessively to undesirably enlarge loads which must be borne by the pump, the motor and the accumulator. Thus, there is apprehension that the durability of the above-mentioned elements deteriorate.
Since the alarm delay circuit is provided to prevent a malfunction in issuing an alarm, there arises another problem in that the cost cannot be reduced.